1. Field of the Invention
The present invention relates to a display apparatus, such as a projection type display apparatus for displaying computer images, video images, etc. on a large screen.
2. Description of the Related Art
Recently, the purposes of use of projection type display apparatuses for displaying computer images in presentations using computers are diversified, and there is a demand for a projection type display apparatus which provides optimum color purity, color balance, illuminance, etc. according to the purpose of use.
FIG. 8 shows the construction of a conventional projection type display apparatus. In FIG. 8, white light emitted from a light source portion 101 is condensed by a reflector 102, passes through fly-eye lenses 103 and 104, a polarization transformation element 105, and a condenser lens 106. Then, it travels by way of a mirror M1 and is separated into three color lights of R, G and B by dichroic mirrors DM1 and DM2, transmitted through condenser lenses 107G and 107B and a relay lens system 107R, transmitted through dichroic filters DF(G) and DF(R), and transmitted through image display elements 108R, 108G and 108B before the color lights of R, G and B are synthesized into one, the image displayed on the image display elements being projected in an enlarged state onto a screen or the like (not shown) by a projection lens 109.
As the light source, a discharge lamp such as a metal halide lamp or a mercury vapor lamp is used. FIG. 9 shows an example of the spectral distribution of a white light source.
As shown in FIG. 9, generally speaking, a white light source has a continuous intensity distribution in the visible light wavelength range of 400 nm to 700 nm. In a projection type display apparatus as shown in FIG. 8, this white light is separated into color lights of R, G and B in a color separation system. At this time, when light component in the wavelength range of 570 nm to 600 nm is taken in the green color light component, the green is turned into yellow, making it difficult to express the pure green color.
Further, when light component in the wavelength range of 570 nm to 600 nm is taken in the red color light component, the red is turned into orange, making it difficult to express the pure red color. In view of this, in the illumination system of the projection type display apparatus, a dichroic filter, etc. are provided on the incident side of the image display elements in addition to the dichroic mirrors DM1 and DM2 to remove the light of the wavelength range of 570 nm to 600 nm so that the light of the wavelength range of 570 nm to 600 nm may not reach any of the image display elements. FIG. 10 shows the spectral distribution of white light synthesized by a dichroic prism DP when light of the wavelength range of 570 nm to 600 nm is removed.
In the construction of FIG. 8, the spectral transmittances of the dichroic mirrors DM1 and DM2 are shown in FIGS. 11 and 12, and the spectral transmittances of dichroic filters DF(G) and DF(R) when light of the wavelength range of 570 nm to 600 nm is removed by providing the dichroic filters DF(G) and DF(R) are shown in FIGS. 13 and 14. Here, there is a technique in which at least one of the dichroic filters DF(R) and the DF(G) can be inserted and removed into and from the optical path, making it also possible to display a color image with no dichroic filter. According to this technique, the dichroic filter is inserted and removed into and from the optical path to thereby vary the purity of the corresponding color to vary the light quantity, whereby it is possible, in a single projection type display apparatus, to select between a color-purity-emphasized (color-reproduction-emphasized) display form (normal mode) and a brightness-emphasized display form.
However, the construction in which the dichroic filter is inserted or removed to thereby vary the purity of the corresponding color to vary the light quantity, has the following problems.
1. When a user of the projection type display apparatus has inserted or removed the dichroic filter into or from the optical path, it is possible for the user to recognize whether the apparatus is in the color-purity-emphasized mode or the brightness-emphasized mode. However, if after this user has inserted or removed the dichroic filter, another user uses the projection type display apparatus to display an image from which it is difficult to recognize whether the apparatus is in the color-purity-emphasized mode or the brightness-emphasized mode, the apparatus might be used in a state not suited for the purpose of use of the other user.
2. When the dichroic filter is removed from the optical path (when it is on standby outside the optical path), the quantity of light impinging upon the image display elements is larger than when the dichroic filter is in the optical path, so that the amount of heat generated at the polarizing plates provided in front of and behind the image display elements also substantially increases, with the result that the temperature in the vicinity of the image display elements rises, thereby adversely affecting the characteristics of the image display elements and the polarizing plates.